This invention relates generally to electronic media storage and retrieval, and more particularly, to an improved method and apparatus for storing and retrieving large amounts of data contained within magnetic tape cartridges.
The use of standardized magnetic tape reels has been supplanted by small, rectangular cartridges such as those used in the IBM 3480 tape subsystem. An example of an automated memory cartridge system for handling tapes such as the IBM 3480 is taught by Moy et al. in U.S. Pat. Nos. 4,928,245, 4,932,826 and 4,864,511, all assigned to the Storage Technology Corporation. The four-by-five inch cartridge used in the IBM 3480 tape subsystem is about one-fourth the size of a standard 10.5-inch reel of magnetic tape, yet it stores up to 20% more data, or a total of 200 million characters. Thus, memory systems that use the IBM 3480 tape subsystem can be relatively efficient with respect to space requirements.
Advances provided by the digital servo control and head technology of such tape subsystems permit low acceleration tape motion, thereby eliminating the need for vacuum columns and capstans. Tape cartridges can also improve data reliability through reduction of both contaminant and handling damage. However, the advantages gained can be nearly offset by the disadvantages accruing from the use of book shelf type storage racks, such as those described in U.S. Pat. No. 4,600,107, and the difficulties which arise in selecting and handling the cartridges.
Book shelf storage racks are used to permit the selection of multiple tapes that can be read. Hence, data from multiple cartridges is randomly accessible and available on-line. In the prior art, the storage and retrieval of such cartridges was for the most part carried out by human operators. Such human intervention not only decreased the reliability of tape selection and replacement, but also prolonged the time from which a data request is made and that data is read by loading a specific cartridge into an available tape drive. While recent improvements to systems utilizing tape cartridges that incorporate magazine type automatic loaders have reduced the time that jobs wait for cartridges to be mounted and subsequently read, many such systems still require operator assistance and are inflexible as to the mounting order once installed within the magazine. It is therefore desirable to provide a randomly accessible storage and retrieval system for magnetic tape cartridges, which are capable of storing large amounts of data, while consuming as little floor space as possible, and at the same time making the data readily available as quickly as possible by minimizing human intervention.
Two prior art attempts at solving these problems are disclosed in U.S. Pat. Nos. 3,938,190 and 4,527,262, in which the systems are adapted for storing information-bearing units or modules in a polygonal configuration. For example, the storage and retrieval system for magnetic tape cassettes described in U.S. Pat. No. 3,938,190 includes a fixed two-dimensional storage array, a fixed processing or play station for extracting information from the units, and a movable selection mechanism having three degrees of freedom for transporting individual modules between the storage and transducing areas. Additionally, the system includes means for automatically sequencing a series of units, as well as a preprocessing station for cuing up individual units for intra-unit accessing.
The information storer and retriever shown in U.S. Pat. No. 4,527,262, on the other hand, includes a plurality of shelves disposed so as to be substantially tangent to a cylinder of reference of a given diameter defining an access corridor which is large enough to accommodate a transport apparatus in the approximate center of the corridor. The transport mechanism includes a carriage that is translatably disposed on a pair of parallel rods to effect X-axis translation of the carriage through the access corridor of the polygonal arrangement, and means for rotating the transport mechanism about the X-axis to select a particular shelf disposed within the polygonal arrangement. Once the appropriate angular position is achieved and the carriage has been translated in the appropriate distance along the X-axis, then the in and out movement or translation perpendicular to the X-axis is ready for activation by appropriate means.
Each of the aforedescribed U.S. Pat. Nos. 3,938,190 and 4,527,262 effectively stores and retrieves a plurality of information bearing media, but both are limited in the amounts of discrete media which may be contained therein. The substantially circular arrays provided increased amounts of data which may be stored per unit floor space, but neither system includes the capability for expansion of storage by interconnecting individual storage units, one with the other, to improve data handling efficiency.
U.S. Pat. Nos. 4,864,511, 4,928,245 and 4,932,826 to Moy et al., and assigned to the Storage Technology Corporation, herein incorporated by reference, disclose expandable automated cartridge memory systems whereby a rotating horizontal arm can be positioned to particular tape addresses. The rotating horizontal arm can move to the location of any stored tape. At the end of the rotating horizontal arm, two hands are mounted to a vertical axis to enable tape cartridges to be transported between storage and transducing positions. Two hands are mounted to the vertical axis in order to provide system redundancy. The vertical axis is mounted to the end of the horizontal arm in order to allow the hands to be positioned at any vertical tier of tape addresses. Performance limitations in such systems have been discovered, in the course of development of the present invention, to result from the use of an unbalanced horizontal arm assembly with a pair of hands and a single vertical axis mounted at one end of the horizontal arm.
The horizontal arm is unbalanced because two hands and a single vertical axis are mounted at one end. As a result, high magnitude moments are generated upon the floor of the robotic system by the unbalanced load rotating at relatively high velocities, and therefore, severe oscillatory motion can result. Because the storage tapes are optimally addressed as quickly as possible, high rotation rates are desirable, but oscillatory motion arising from an unbalanced horizontal arm can lead to difficulties.
Oscillatory motion can prevent the hands from quickly transferring tapes between storage and transducing positions. If the horizontal arm assembly is oscillating even slightly, then the vertical axis and hands at the end of the arm are moving with sufficient amplitude so that the hands are incapable of grabbing the tapes. As a result, the overall system response must be slowed sufficiently to enable the amplitude of the oscillatory movement to diminish before the hands attempt to grab the tapes. Otherwise, the tapes can be dropped and subsequently not loaded to the transducing unit. Dropped tapes must be attended to by human intervention, and therefore, the tape loading process slows down even more considerably.
The large moments exerted by the robotic structure causes the floor of the automated memory cartridge system to bend which can cause the entire automated cartridge system unit to shift over time. Such bending and spatial displacements ultimately affect system reliability. For example, a rotational servo controls the rotation of the horizontal arm. When oscillatory motion is present, the bearings in that servo can fail prematurely.
The use of a single unbalanced horizontal arm requires that two hands be mounted at the single end to achieve the desired system redundancy. Thus, the weight of two hands is concentrated at a single end of the revolving horizontal arm, thereby producing greater moments which increase the amplitude of the undesired forces exerted on the robot and its support structure, and further creates greater stress on the servo responsible for rotational motion.
Finally, with the single horizontal arm system, the arm could rotate up to 180.degree. to locate any particular tape, thereby further reducing the tape access speed.